Field of the Invention
The invention relates to a one-piece optical element for focusing approximately collimated rays.
Description of the Background Art
Optical elements for ray focusing, which diffract collimated input rays around an optical axis into output rays, which overlap in a focal region, are known from the state of the art. It is possible thereby to change a relatively low average input irradiance, distributed over a relatively broad entrance pupil, into a relatively higher average output irradiance, concentrated in a relatively narrow cross section of the focal region.
The document US 2010/0309566 A1 describes an optical system with at least two reflaxicons made of a solid, light-transmitting material. Each reflaxicon has an inner cone-shaped surface and an outer truncated-cone-shaped surface, which are formed centered to one another along an optical axis and reflective.
Known from the prior art is the use of such optical elements for ray focusing in order to exceed a minimum irradiance only within the focal region, above which certain physical effects such as polymerization, optical perforation, or melting of solid materials are initiated. Thus, it is possible to work on materials or biological tissue in a spatial section, relatively sharply delineated by the focal region.
Optical elements with which an input bundle of rays having a predetermined cross section can be focused on an especially narrow focal region are advantageous both to achieve especially high irradiances and for especially precise treatment. For example, aspheric lenses are known for this purpose from the prior art; based on the laws of geometrical optics, said lenses can be formed so that any input rays, running parallel to the optical axis, for light of one wavelength can be diffracted into output rays, which intersect in a focal point, located at the distance of the focal length from the exit side principal plane of the aspheric lens on the optical axis.
With consideration of wave optical effects, however, no focal point of infinitesimally small extension can be achieved with aspheric lenses of this type as well and also for monochromatic light, but only a focal region of finite extension, which typically is given by the diameter of the Airy disk
            d      Airy        =          1.22      ·              λ                  n          ·                      sin            ⁡                          (              α              )                                            ,where λ is the wavelength of the monochromatic light, n is the refractive index of the medium surrounding the lens, and α is half of the exit side aperture angle of the aspheric lens.
The systems and methods according to the prior art reduce the extent of the focal region by increasing the numerical aperture. When the cross section of the entrance pupil remains the same, an increase in the numerical aperture can be brought about according to the prior art by reducing the focal length, and therefore also by reducing the working distance between the front surface of the lens and the material to be treated. According to the prior art, an increase in the numerical aperture can also be brought about by the use of an immersion liquid between the lens and the material to be treated, which liquid has a higher refractive index than air.